Cable TV (CATV) systems were initially deployed as video delivery systems that, in their most basic form received video signals at a cable head end, processed the signals for transmission, and broadcast them to homes via a tree-and-branch coaxial cable network. In order to deliver multiple TV channels concurrently, early CATV systems assigned 6 MHz blocks of frequency to each channel and Frequency Division Multiplexed (FDM) the channels onto the coaxial cable RF signals. Electrical amplifiers were inserted along the transmission path to boost the signal, and splitters and taps were deployed to deliver the signals to individual homes.
As the reach of the systems increased, the signal distortion and operational costs of long chains of electrical amplifiers became problematic, hence over time larger segments of the coaxial cable in the tree-and-branch transmission network were replaced with fiber optic cables, creating a Hybrid Fiber Coax (HFC) network. The HFC network uses optical fiber to deliver the RF broadcast content from the head end to the remaining segments of coaxial cable in the network neighborhood transmission network, which in turn delivers it to the subscribers. Optical nodes in the network acted as optical to electrical converters to provide the fiber-to-coax interfaces.
Over the years, HFC is continually evolving to push fiber deeper in to the network. Eventually, it will reach the point where it becomes a Fiber to the Premise (FTTP) architecture, but this may take decades at an economical pace. FTTP is happening today in new Greenfield deployments, yet there are significant operational challenges to make this transformation in existing HFC infrastructure, a.k.a. Brownfields.
Simultaneously, the HFC network is evolving to deliver an increasing amount of content to subscribers, as well as provide data (e.g., Internet) services at ever-higher speeds. Such data services are IP packet-based services, but are propagated on the HFC network as additional frequency blocks that use FDM to share the spectrum along with video services. Unlike broadcast video, each IP stream is unique. Thus, the amount of spectrum required for data services is a function of the number of data users and the amount of content they are downloading. With the rise of the Internet video, this spectrum is growing at 50% compound annual growth rate and putting significant pressure on the available bandwidth. Pressure on the available bandwidth has further increased with the advent of narrowcast video services such as video-on-demand (VOD), which changes the broadcast video model as users can select an individual program to watch and use VCR-like controls to start, stop, and fast-forward. In this case, as with data service, each user requires an individual program stream.
Unlike broadcast video, data services require a two-way connection. Therefore, the cable plant must provide a functional return path, i.e. data communication between the CATV head end and subscribers includes a downstream path that delivers video and data to subscribers, along with a return path that delivers data from the subscribers to the head end. To prevent interference between the upstream and downstream signals when transmitted over HFC network, separate ranges of bandwidth were dedicated to these upstream and downstream signals respectively, such that a smaller, low-frequency range of the total transmission spectrum (for the upstream signal) was “split” from a larger, higher frequency range (for the downstream signal). As can easily be appreciated, as more video content and faster data services are provided via the HFC network over time, the “split” between the upstream and downstream paths must change. Historically, HFC systems have supported several different splits, including 42, 55 and 65 MHz splits. The DOCSIS 3.0 standard introduced a 85 MHz split, but this split not been widely deployed due to the difficulties of moving legacy services (e.g. STB control channel, FM channels) from existing 54-108 MHz spectrum reserved for downstream content. Moreover, the DOC SIS 3.1 standard further contemplates a significant increase in upstream spectrum, and associated capacity, with the option of a 204 MHz upstream split with the corresponding downstream spectrum starting at 258 MHz. This however exacerbates the difficulties arising from supporting legacy downstream services in the 54-258 MHz range.
Rather than migrate to new architectures, such as fiber-to-the-premises (FTTP) where fiber replaces all portions of the CATV network, many existing CATV providers have tended to squeeze as much content and services as possible over the existing CATV architecture. However, the capacity of the existing HFC architecture is limited, and this solution will be adequate for only so long.
What is desired, therefore, are improved methods and systems for transmitting the breadth of content contemplated by the DOC SIS 3.1 standard over an HFC network while simultaneously providing support for legacy downstream service.